One of the key challenges facing modern industrialized society is the limited sources of fossil fuel that are accelerating the development of renewable sources of fuel. Several new sources have been pioneered during the past 20 years, and one of the more attractive sources is fuel that is derived from biomass, referred to as biofuel. It has been speculated that up to 30 percent of current transportation fuel will be replaced by biofuel in the next 5 to 10 years. Although biofuel has been successfully introduced into the marketplace, certain properties of biofuel have restricted the use of larger quantities of biofuel for diverse transportation applications.
The most successful class of biofuels is biodiesel, which is produced from crop oils or animals fats. Unfortunately, biodiesel will not work as a diesel fossil fuel replacement for an aviation turbine due to limitations in its temperature dependent properties at the atmospheric temperatures experienced during flight. For example, military specifications require that aviation turbine fuel be completely resistant to the formation of solid crystals at temperatures as low as −47° C., which corresponds to an altitude of 9500 meters or about 31,000 feet. Unacceptably, canola methyl ester (“CME”) and soy methyl ester (“SME”) biodiesel have typical cloud points of 1.0° C. and 3.0° C., respectively and pour points of −9.0° C. and −3.0° C., respectively (Graboski, M. et al., Prog. Energy Combust. Sci, 24, 1998, 125-164). Similarly, biodiesel will not work as a fossil fuel replacement for diesel engines in cold temperatures. The chemical properties of biofuels currently in the marketplace are such that solids form in the fuel solution at cold temperatures, referred to “clouding” zones. Fuel clouding can cause fuel filters, carburetors or any small orifices to become clogged resulting in significant damage and repair expense and possibly resulting in death of the operator and passengers.
Three general classes of strategies have been investigated to overcome these cold flow limitations. These involve one or more methods to “winterize” the fuel by physical removal of the chemical components that solidify above the target freeze point, use of additives to inhibit solidification, or chemical manipulation of the fuel composition to modify the properties of the fuel.
One type of winterization process for biodiesel fuel involves first chilling or partially distilling the fuels followed by chilling and subsequently filtering out any precipitated solids (U.S. Patent Publication No. 2004/0231236). In this method, high melting point components of biodiesel are removed to decrease the fuel's freezing point, making it comparable to petroleum-based diesel fuels. This technique typically requires refrigeration of the biodiesel over lengthy time periods, e.g. 16 hours or longer, at the target cold flow temperature, followed by filtration of the solid crystals. The removal of partially solidified triglycerides reduces the cloud point and pour point of the biodiesel; however, the composition does not conform to the desired standards. Further, this method is time consuming and commercially expensive.
In yet another winterization approach, the biodiesel is subjected to a series of fractionation processes where material of lower volatility is separated out of the fuel. Lower volatility material tends to have a higher freezing point. By removing this material, the cold flow properties of the biofuel are enhanced. This method yields a pour point between −15° C. and −24° C. However, this temperature range is above the range required for aviation fuel or for a diesel fuel for very cold regions. This winterization method gives very low yields (<30% by weight), decreased cetane numbers (a measure of the quality of a fuel for diesel and turbine engines), and reductions in resistance to oxidation causing loss in combustion quality that may lead to engine durability problems as well as an increase in harmful exhaust emissions (Dunn, R. et al., “Low Temperature Properties of Triglyceride based Diesel Fuels”, J. American Oil Chemists Soc, 72, 1995).
The second general class of methods to improve the cold flow characteristics of crop oil-based fuels is the addition of compounds that act to improve cold flow properties. Commercial additives developed for improving cold flow of conventional turbine and diesel fuels are largely ineffective for biodiesel. These are chemicals added in very small quantities (0.1-0.2% by volume) that can lower the cloud point. Typically they do so primarily by bonding to frozen molecules when the fuel falls below the cloud point, thus preventing those molecules from bonding/cross linking with other frozen molecules. Therefore, the additives are beneficial with respect to inhibiting nucleation and crystalline growth of biodiesel molecules. In general, the properties of these additives are inadequate because they primarily affect the pour point rather than the cloud point or have a minimal impact on cloud point. The cloud point is recognized to be a more critical property than the pour point for improving low-temperature flow properties because it is a low-temperature operability indicator (Dunn, R. O. “Alternative Jet Fuels From Vegetable Oils”, American Society of Agricultural Engineers, Vol. 44(6), 2001, pp. 1751-1757).
The third class of methods to reduce the cold temperature properties of a biofuel is to chemically modify the crop oil or biodiesel. A number of inventions are based on modifying the transesterification process that converts free fatty acid oil into biodiesel. The most common approach is to use branched chain alcohols to esterify the crop oil such as isopropanol, isobutanol, and 2-butanol rather than methanol (Lee, I. et al., “Use of Branched-Chain Esters to Reduce the Crystallization Temperature of Biodiesel”, Journal of the American Oil Chemists' Society, 72, 1995, 1155-1160). Branched esters have lower freezing points in the neat form and have been shown to improve the cloud point and pour point of biodiesel fuels. For example, isopropyl soyate has a cloud point of −9° C. and 2-butyl soyate has a cloud point of −12° C. In comparison, the cloud point of methyl soyate is 0° C. However, no esterification process has been developed that can achieve the necessary cold flow properties for aviation turbine fuels.
In another variation of this method, the transesterification reaction is carried out in methanol and/or ethanol but with the addition of methyl or ethyl acetates of fatty acids and an inert solvent (U.S. Pat. Publication No. 2003/0167681). This is followed by separation and blending steps to produce a biodiesel with improved properties at low temperatures. However, the freezing point ranged from −10° C. to −17° C. and is still not an adequate replacement for JP-8 aviation fuel. Detailed specifications for JP-8 can be found in MIL-DTL-83133E.
While all of these inventions offer improvements over untreated crop oil for temperature performance of a biofuel product, none of these methods provides for a biofuel product that meets commercial low temperature requirements. In addition, these methods suffer from inherent disadvantages that limit their economic feasibility in the marketplace.